1. Field of the Invention
The present invention relates to a stirring apparatus for stirring a ferric chloride etching waste fluid containing heavy metals and masses of metallic iron in order to mix them with each other and to thereby precipitate the heavy metals in the waste fluid with a high efficiency.
2. Description of the Prior Art
A ferric chloride solution has been extensively used as an etching solution for etching metallic plates comprising steel, copper, stainless steel, nickel alloy or the like. When this kind of etching solution is repeatedly used, heavy metals (nickel, chromium, copper, manganese, iron and the like) dissolved therein are accumulated and concentrated therein and trivalent iron ions are finally reduced to divalent iron ions, so that ferric chloride in the solution is changed into ferrous chloride, with the result that the etching power of the solution declines and the solution eventually turns to a waste fluid.
Heretofore, as a treating process for the waste fluid, there is taken a means for adding a solution of an alkali such as lime [Ca(OH).sub.2 ] to the waste fluid in order to neutralize the latter and to thereby precipitate the heavy metals in the form of hydroxides. In this process, however, all of the heavy metals, including iron, are precipitated, and it is therefore difficult to separate iron from the other heavy metals and when one attempts to then dispose of the thus formed precipitate of the heavy metals, or sludge, many problems of "enviornmental pollution", are created. In particular, the recovery of ferric chloride is not possible in connection with this process and this fact is not economical.
As a consequence, there is considerable demand for a novel technique which permits one to recover the ferric chloride solution and recycle same a number of times.
Heretofore, in order to recover the ferric chloride etching waste fluid, a method has been used which comprises adding masses of metallic iron to the ferric chloride etching waste fluid, precipitating the heavy metals, exclusive of iron, by utilizing the difference in a difference of ionization potential between iron and the other heavy metals, filtering out the precipitated heavy metals, and blowing a chlorine gas thereinto so as to change divalent iron ions into trivalent iron ions. Where a great deal of nickel is present in the waste fluid, however, this method entails the following problem: Since it has a strong affinity for iron, nickel is liable to precipitate together with the iron, with the nickel then adhering to iron surfaces. This adhered nickel then renders the iron passive, so as to undesirably halt the exchange precipitation reaction between iron and the other heavy metals having a low tendency toward ionization.
One measure which can be used to combat this problem is to vigorously stir a mixture of the waste fluid and the iron masses, whereby the iron masses are caused to collide with each other and nickel is thus peeled off from the iron surfaces so as to expose fresh iron surfaces. However, even where this stirring operation is carried out with the specific intention of obtaining such an effect, conventional stirring methods can scarcely provide the desired effect on occasion. Thus, there has been a demand for novel stirring means which are capable of assuring such an effect.
Furthermore, in order to accelerate the precipitation reaction of the heavy metals, it is necessary to maintain the mixture of the waste fluid and the metallic iron masses at an elevated temperature. However, the use of conventional heating system can at times give rise to local or transient overheating which can lead to an explosive reaction, or the leakage of liquid or a gas as a result of a rapid pressure increase in the reaction container, due to problems with the material quality, such as a crack in the reaction container owing to temperature rises in the waste fluid. There is thus also a demand for elimination of these drawbacks.